The present invention relates to fuel cells, and more particularly to fuel cells that employ regenerative braking.
Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have also been proposed for use in vehicles as a replacement for internal combustion engines. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and oxygen (O2) is supplied to the cathode. The source of the hydrogen is typically pure hydrogen, reformed methanol, or other reformed hydrocarbon fuels.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (exe2x88x92). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the membrane. The electrical load is typically a motor that drives the wheels of the vehicle or storage batteries. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (exe2x88x92) are taken up to form water (H2O). Therefore, fuel cell vehicles have little or no emissions.
Internal combustion engine vehicles and hybrid vehicles sometimes employ regenerative braking to improve the efficiency of the vehicle. In non-regenerative braking vehicles, the torque produced by the brakes causes friction that slows the wheels of the vehicle. The friction creates waste heat that increases the temperature of the brakes. Regenerative braking devices convert mechanical brake torque that occurs during vehicle deceleration into power. The energy that is produced by the brake torque is typically used to recharge a battery pack that powers vehicle accessory loads such as the lights, radio, pumps, air conditioner, fans, and other devices.
In U.S. Pat. No. 4,489,242 to Worst, a vehicle power system includes an internal combustion engine and a regenerative braking device that charges a battery pack. The battery pack powers one or more vehicle accessories such as vehicle lights, power steering and brake pumps, air conditioner, radiator fan, water pump, etc. In U.S. Pat. No. 5,345,761 to King et al., regenerative braking is used to power a high-voltage, electrically-heated catalyst that treats the exhaust gas of an internal combustion engine. In U.S. Pat. No. 6,122,588, regenerative braking is used to supply power to increase fuel efficiency and/or to power various electrical loads such as vehicle accessories.
Regenerative braking is generally provided by a motor/generator that opposes the rotation of the wheels by applying a negative or regarding torque to the wheels of the vehicle. Because the negative torque decelerates the vehicle and is often used to assist the brakes, regenerative braking systems generally reduce the wear on the brakes of the vehicle, which reduces maintenance costs.
Because fuel cell vehicles are relatively new in the automotive arena, current fuel cells do not produce as much power as internal combustion engines. Fuel cell vehicles are also more expensive than internal combustion engines. Before widespread acceptance of fuel cells will occur, these performance and cost issues must be resolved. The performance of the fuel cell is related to the weight of the fuel cell. Because of the increased weight and cost of battery packs and DC/DC converters that are required in regenerative braking systems, fuel cell have not implemented regenerative braking systems.
A regenerative braking system and method for a batteriless fuel cell vehicle includes a fuel cell stack, an ancillary load, and a regenerative braking device that is coupled to at least one wheel of the vehicle. The regenerative braking device powers the ancillary load when the vehicle is coasting or braking. The fuel cell powers the ancillary load when the vehicle is accelerating or at constant velocity.
In other features of the invention, the regenerative braking system includes an air compressor. The regenerative braking device dissipates power in the air compressor when the vehicle is traveling downhill to provide brake assistance. A bypass valve has an inlet connected to the air compressor. When the vehicle is traveling downhill, the air compressor is run at high airflow and high pressure to create an artificial load. The bypass valve is modulated to adjust the artificial load of the air compressor.
In still other features of the invention, the regenerative braking device is an electric traction system. A back pressure valve is connected to a cathode of the fuel cell stack. The back pressure valve protects the fuel cell stack from the high airflow and pressure. A controller controls a brake torque of the regenerative braking device as a function of vehicle speed and modulates the bypass valve to vary the artificial load.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.